1. Field of Invention
The present invention relates to illumination devices that use integrators. More specifically, the invention relates to an illumination device of a color-recapture-type.
2. Description of Related Art
In a related art illumination device, such as that disclosed in Japanese Laid-Open Patent Application Publication No. Hei6-88953, a front mirror is disposed at the front of a paraboloid of revolution type reflecting mirror, which converges illumination light from a lamp light source ahead of the light source, and reflects a part of the illumination light from the lamp light source. Also, a center mirror to collimate the light reflected at the front mirror so as to reflect it in the forward direction is provided.
The related art also includes an illumination device which relays only a part of illumination light from a light source, the part being incident on an integrator, to a optical system lying ahead so as to use it as illumination light. For example, in order to display a color image with an image display apparatus having an optical system to modulate light of a single color, such as a so-called single panel projector, the image display apparatus has a color wheel (a color filter) to selectively transmit or absorb white light so as to separate it into luminous flux of three primary colors in a time-division manner by using a rotary color filter, and the integrator to make the intensity distribution of the light uniform; color modulation of the luminous flux of the respective colors is performed by a light valve such as a micro-mirror device; and the modulated luminous flux are projected onto a screen so as to generate a composite color image. In the projector of this system (optical system), since the integrator is disposed behind the color filter, when the color separation of the white light is performed by the filter, a part of the light which does not pass through the filter is reflected thereat and discarded. This is one of the causes of the reduced light utilization efficiency.
The related art also includes a color-recapture-type illumination device having a structure in which the integrator is disposed at the incident side of the color filter, that is, between the light source and the color filter, and the color filter is a color wheel formed of a combination of translucent dichroic films (filters) so as to be provided in a suitable shape, such as a spiral shape, each transmitting light corresponding to its specific color and reflecting light corresponding to the other colors. In this color-recapture-type illumination device, each dichroic film of the corresponding color performs color separation and reflects light which does not pass therethrough towards the integrator. With this structure, although about a fraction of the illumination light from the light source passes through the integrator in one pass and is output to one of the dichroic films corresponding to one of the colors of the color wheel, the light which does not pass through the color filter and is reflected thereat is returned to the integrator without being discarded. Accordingly, the light which does not pass through the color wheel transmits in the integrator and is again made incident on one of the dichroic films of the color filter. On this occasion, when the incident light encounters another one of the dichroic films, the incident light is transmitted through and output from the other dichroic film. As a result, the illumination light from the light source can be used without reducing the light intensity, the light loss decreases, and thus the light utilization efficiency can be enhanced in comparison to the above related art method to perform a color display, thereby achieving a bright color display.